component/default: system gcc can be used only if all gcc, g++ and gfortran exist.

component/defaults.cfg

@@ -42,13 +42,22 @@ min_version = 5.4
 init =
   import os, subprocess
   parse_version = lambda ver: tuple(map(int, ver.strip().split('.')))
+  version_list = []
+  for command in ('gcc', 'g++', 'gfortran'):
     try:
-    current = subprocess.check_output(('gcc', '-dumpfullversion'),
+      version = subprocess.check_output((command, '-dumpfullversion'),
                                         stderr=subprocess.STDOUT,
                                         universal_newlines=True).strip()
     except subprocess.CalledProcessError: # BBB: old GCC
-    current = subprocess.check_output(('gcc', '-dumpversion'),
+      version = subprocess.check_output((command, '-dumpversion'),
                                         universal_newlines=True).strip()
+    except FileNotFoundError:
+      version = None
+    version_list.append(version)
+  if None in version_list:
+    current = '0.0.0'
+  else:
+    current = sorted(version_list, key=parse_version)[0]
   self.system_version = current
 # If we're still going to use the same GCC,
 # the conditions have no impact on the dependant parts.